The present invention relates to a structure for an electronic endoscope, and in particular to a structure to be installed in a front end part of an electronic endoscope which is inserted into a body cavity for endoscopic observation and which is provided with a solid-state imaging element for capturing endoscopic images.
(1) In order to improve insertion ability of an endoscope, i.e. to reduce pain to a patient during endoscopic observation, smaller the size of a front end part of the endoscope is better. An electronic endoscope needs to equip, in its front end part, with a solid-state imaging element for capturing endoscopic images obtained by an objective lens group as well as arrangements for electrically connecting the solid-state imaging element to a wiring circuit board and for providing necessary electric insulation, and thus the front end part of the electronic endoscope tends to become bulky. Accordingly, an object of the present invention is to provide a structure for a front end part of an electronic endoscope, which can reduce the size of the front end part of the electronic endoscope to thereby improve the insertion ability thereof.
(2) In order to avoid unwanted marginal rays from reaching a rectangular light receiving surface of a solid-state imaging element and thus to improve obtained endoscopic images, Japanese laid-open patent publication No. Hei-3-2813 proposes providing a rectangular shading mask for blocking the unwanted marginal rays inside the objective optical group or on the surface of the first foremost lens.
However, when a shading mask is disposed inside the objective optical group, not only does the assembly of the objective optical group become troublesome, but since the unwanted marginal rays can still enter up to the position of the shading mask inside the objective optical group, flare, etc., may occur due to the reflection or the like before marginal rays reach the shading mask.
In case where the shading mask is disposed on the surface of the first lens, the shading mask protrudes from the surface of the first foremost lens, making it difficult to remove foul fluids, etc., that have become attached to the surface of the first foremost lens and thus leading to significant degradation of the picture quality of the observed image.
Accordingly, another object of the present invention is to provide a structure for a front end part of an electronic endoscope, which can positively avoid entry of unwanted marginal rays into an objective optical group or system without the use of a shading mask so that satisfactory picture quality without flare, etc., can be obtained.
In an endoscope, a first lens (a foremost lens such as a cover glass, a convex lens) of a lens group is generally fitted onto an observation window, which is the entrance window through which the optical image enters the objective lens group.
The observation window is generally provided at a front end part of the endoscope. During assembly, a lens frame in which objective lens or lenses other than the first lens is installed is assembled to the rear portion of the front end part on which the first lens has already been mounted.
Therefore, dust or debris is like to be attached to the rear face of the first lens before the lens frame is installed at the prescribed position inside the front end part, and the work of removing such debris from the rear side of the front end part is extremely difficult.
Accordingly, another object of the present invention is to provide a structure for a front end part of electronic endoscope, which can be assembled readily and which can eliminate the problem of dropping of debris onto the rear face of the first optical member during the process of installing the objective optical group into the front end part of the endoscope.